Percutaneous disc clearing device

ABSTRACT

A discectomy tool comprising:
         a) a cannula having an outer surface having a longitudinal bore therein, a proximal end and a distal end;   b) a steering wire disposed in the longitudinal bore;   c) a flexible, hollow transmission shaft disposed in the cannula, the shaft having a throughbore, a proximal end portion, a distal end portion and an outer surface having a thread extending therefrom;   d) an irrigation source fluidly connected to the throughbore;   e) a cutting tip attached to the distal end portion of the transmission shaft.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 14/674,310, filed on Mar. 31, 2015, the entire contents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Lumbar interbody fusion is a surgical procedure that is often performed up on instabilities within the lumbar spine. These instabilities are either the result of a medical condition like degenerative disc disease (DDD) or a vertebral bone fracture, or the result of a surgical decompression procedure treating stenosis, in which soft tissue and/or bony structures around compressed neural structures in the spine are removed. To achieve an acceptable interbody fusion, the existing disc needs to be removed (discectomy), and bone graft and/or an implanted cage is put on its place.

There are several approaches through which the disc clearing and cage insertion steps can be performed, each with its benefits and risks. One of the most popular is the transforaminal approach, commonly used in a transforaminal lumbar interbody fusion (TLIF).

In a TLIF approach, the creation of an access window that is necessary to insert the cage (implant) normally requires the removal of parts of the facet joint, and takes time. Moreover, the iatrogenic trauma produces by this procedure induces a significant amount of destabilization and recovery time.

If there were a possibility of performing the whole interbody fusion procedure through a percutaneous or endoscopic working channel, the iatrogenic trauma, risk of neural damage during the access and fusion procedure, surgery time and most probably the infection risk might be significantly reduced.

The current standard disc clearing step in a fusion surgery is a very manually-intensive procedure, and requires about sixty instrument passes close to the dura and nerve roots, in which instruments like rongeurs and curettes transport the excised material out of the patient. Today, it is difficult to perform such clearing through a percutaneous working channel of 4-12 mm outer diameter. Therefore, it is a goal to eliminate the need for continuous instrument passes close to the nerves.

Problems associated with convention discectomy devices include inefficient tissue cutting, clotting and inability to be steered.

The following references disclose discectomy tools: U.S. Pat. Nos. 5,285,795; 4,863,430; US2011-054507; US2010-076476; US2013-0103067; US2008-0015621; US2012-0221007; US2005-0090848; US2012-0209273; US2006-0206118; U.S. Pat. Nos. 5,540,706; 6,053,907; 5,591,187; 4,646,738; US2002-0138020; US2007-0055259; U.S. Pat. No. 5,529,580; US2007-0149975; US2003-0191474; US2010-0151161; U.S. Pat. Nos. 4,678,459; 5,195,541; US2004-0127992; US2003-0171744; US2010-0161060; U.S. Pat. Nos. 8,784,421; 6,468,289; WO 09-033207; WO 2014-100761; and U.S. Pat. No. 8,585,726.

SUMMARY OF THE INVENTION

There is provided a discectomy device that can be introduced through a small working channel (diameter 4-12 mm). The working channel is introduced through a variety of possible approaches (e.g. ELIF/TLIF/lateral) so that the distal end of the working channel extends a few millimeters into the annulus of the (lumbar) disc. When the device is fully introduced into the working channel, its drill-like tip is located fully within the disc (i.e., this tip exceeds the distal end of the working channel).

Preferably, the drill-like tip of the device can rotate and is powered. The tip's geometry enables cutting and detaching disc material (nucleus pulposus and inner annulus).

More preferably, the cutting tip is that disclosed in U.S. Ser. No. 13/836,889, filed Mar. 15, 2013, entitled “TOOLS AND METHODS FOR TISSUE REMOVAL,” now issued as U.S. Pat. No. 9,603,610, the specification of which is incorporated by reference in its entirety.

The device has sufficient flexibility to be steered actively and to sweep and detach disc material at locations. In contrast, a relatively stiff instrument—if introduced through the same limited-flexibility working channel—could never reach those locations. The device preferably has the necessary steering stability and bending radius to reach and clear a sufficient amount of disc area so as to permit the subsequent placement of bone graft material therein in order to reach a stable fusion.

In some embodiments of the present invention, the discectomy device has a combination of the following design elements:

-   -   a cutting blade geometry disclosed in US 2014-0276840), the         specification of which is incorporated by reference in its         entirety;     -   a thin-walled, steerable outer cannula steered by “push-pull”         dovetail-steering—strips;     -   a flexible and torque-transmitting drive shaft, containing a         flexible auger-based disc tissue-transport system;     -   central irrigation provided through the drive shaft and exiting         at the cutting blade to prevent tissue-clotting at the blade or         along the auger; and     -   suction means to support auger for transporting the mix of         irrigation solution and disc material.

The present invention helps to improve the discectomy aspects of the current standard lumbar interbody fusion procedure by enabling the disc clearing step to be performed percutaneously or through an endoscopic working channel, and by enabling the disc clearing step through a standard or mini—open approach (TLIF, ELIF/lateral approach) to be performed automated, safer and faster than with today's standard manual tools.

In some embodiments of the present invention, the percutaneous disc clearing tool can be inserted through a straight or curved, rigid percutaneous working channel (inner diameter range about 4-12 mm, ideally about 5-7.5 mm), having a steerable disc removal member exiting the working channel and being located fully or partly within the vertebral disc.

The disc removal tool can be manually driven or powered, and it has the ability to detach (rupture or cut) nucleus as well as annulus material within the intervertebral disc. Such disc removal mechanisms have been previously described.

The disc removal tool can be actively steered, so that it can sweep within the disc, and detach disc material at locations where a stiff instrument (if introduced through the same working channel) could never reach. The disc removal member has the necessary steering stability and bending radius to reach and clear a sufficient amount of disc area, while being introduced and operated through a rigid and straight working channel.

The cut material can be transported out of the disc, leaving a cavity in the disc that is big enough to allow interbody stabilization & fusion.

Therefore, in accordance with the present invention, there is provided a discectomy tool comprising:

-   -   a) a cannula having an outer surface having a longitudinal bore         therein, a proximal end portion and a distal end portion;     -   b) a steering wire longitudinally contacting the cannula and         extending in the direction of the longitudinal bore;     -   c) a flexible, hollow transmission shaft disposed in the         cannula, the shaft having a throughbore, a proximal end portion,         a distal end portion and an outer surface having a thread         extending therefrom;     -   d) an irrigation source fluidly connected to the throughbore;     -   e) a cutting tip attached to the distal end portion of the         transmission shaft;

Also in accordance with the present invention, there is provided a discectomy tool comprising:

-   -   a) a cannula having an outer surface having a longitudinal bore         therein, a proximal end and a distal end;     -   b) a transmission shaft having a proximal end portion and a         distal end portion;     -   c) a rotatable cutting tip attached to the distal end portion of         the transmission shaft; and     -   d) a flexible helical auger having a proximal end portion, and         intermediate portion and a distal end portion, wherein the         intermediate portion is loosely wrapped around the flexible         torque transmission shaft,         wherein the transmission shaft and auger are adapted to rotate         within the longitudinal bore of the cannula, wherein the cutting         tip extends out the bore at the distal end of the cannula.

Also in accordance with the present invention, there is provided a discectomy tool comprising:

-   -   a) a cannula having an outer surface having a longitudinal bore         therein, a proximal end and a distal end;     -   b) a transmission shaft having a proximal end portion and a         distal end portion;     -   c) a rotatable cutting tip attached to the distal end portion of         the transmission shaft; and     -   d) a flexible helical auger having a proximal end portion, a         hollow intermediate portion and a distal end portion, wherein         the proximal end portion of the auger is connected to the torque         transmission shaft and the distal end portion of the auger is         connected to the cutting tip,         wherein the transmission shaft and auger are adapted to rotate         within the longitudinal bore of the cannula, wherein the cutting         tip extends out the bore at the distal end of the cannula.

Also in accordance with the present invention, there is provided a discectomy tool comprising:

-   -   a) a cannula having a proximal end, a distal end, and an outer         surface having a first longitudinal recess therein;     -   b) a transmission shaft disposed in the cannula, the shaft         having a proximal end portion and a distal end portion;     -   c) a cutting tip attached to the distal end portion of the         transmission shaft;     -   d) a first steering element disposed in the first longitudinal         recess of the outer surface.

Also in accordance with the present invention, there is provided a discectomy tool comprising:

-   a) a cannula having a proximal end, a distal end, and an outer     surface; -   b) a transmission shaft disposed in the cannula, the shaft having a     proximal end portion and a distal end portion; -   c) a cutting tip attached to the distal end portion of the     transmission shaft; -   d) first and second steering elements bilaterally and longitudinally     disposed on the outer surface of the cannula.

Also in accordance with the present invention, there is provided a discectomy tool comprising:

-   -   a) a cannula having a proximal end portion, a distal end         portion, and an outer surface; the cannula having a first         longitudinal face and a second opposed longitudinal face,         wherein the first longitudinal face has a plurality of         alternating, opposed transverse cutouts therealong to form a         first substantially square wave of the first longitudinal face,         (and preferably, the second longitudinal face has a plurality of         alternating, opposed transverse cutouts therealong to form a         second substantially square wave of the second longitudinal         face)     -   b) a transmission shaft disposed in the cannula, the shaft         having a proximal end portion and a distal end portion;     -   c) a cutting tip attached to the distal end portion of the         transmission shaft.

DESCRIPTION OF THE FIGURES

FIG. 1 discloses an embodiment of a discectomy tool of the present invention.

FIGS. 2 and 3 disclose a distal portion of an embodiment of a discectomy tool of the present invention.

FIGS. 4A-4B disclose embodiments of a transmission shaft of the present invention.

FIGS. 5A-F disclose approaches used by a discectomy tool of the present invention in a disc space.

FIGS. 6-8 disclose embodiments of bent transmission shafts of the present invention.

FIGS. 9-11B disclose embodiments of cannulae of the present invention.

FIG. 11C discloses a cross-sectional view of a cannula embodiment of the present invention.

FIG. 11D discloses a side view of a cannula embodiment of the present invention, revealing a first longitudinal face of the cannula.

FIGS. 12-13 disclose cannulae with longitudinal steering elements.

FIGS. 14-15 disclose cross-section of cannulae of the present invention with embedded steering elements.

FIGS. 16-17 disclose cannulae with longitudinal steering elements.

FIGS. 18-19 disclose cannulae with dovetail notches for holding embedded steering elements.

FIGS. 20-22 disclose cannulae in which the steering elements are metallic/polymeric tapered strips.

FIG. 23 is a prior art Archimedes pump.

FIG. 24 is a tool of the present invention having integrated irrigation flow.

FIG. 25 discloses an embodiment of a helical spiral transmission shaft of the present invention.

FIG. 26 discloses an embodiment of a helical spiral loosely connected to a transmission shaft of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-4B, there is provided a discectomy tool comprising:

-   -   a) a cannula 11 having an outer surface 13 having a longitudinal         bore therein, a rigid proximal end portion 2 and a flexible         distal end portion 3;     -   b) a steering wire 4 longitudinally contacting the cannula and         extending in the direction of the longitudinal bore;     -   c) a flexible, hollow transmission shaft 5 disposed in the         cannula, the shaft having a throughbore, a proximal end portion         19, a flexible 6 distal end portion and an outer surface 21         having a flexible thread 7 extending therefrom;     -   d) an irrigation source fluidly connected to the throughbore;     -   e) a cutting tip 8 attached to the distal end portion of the         transmission shaft; and     -   f) a drive/steer/irrigation handle 1.

The invention is not limited to any particular approach trajectory of the working channel. For example, if a certain approach/trajectory offers an advantage in a given situation, the approach can be chosen accordingly. For example, and now referring to FIGS. 5A-5F, the surgeon may desire to use the tool in the following non-limiting approaches:

-   -   a) a flat angle extraforaminal approach (FIG. 5A);     -   b) a steep extraforaminal approach (FIG. 5B);     -   c) a translaminar approach (FIG. 5C);     -   d) a transformainal approach (FIG. 5D);     -   e) a far lateral approach (FIG. 5E); and     -   f) an anterior approach (FIG. 5F).         In order to increase the cleared volume of the disc, the tool         can be serially inserted from multiple approaches, e.g. from two         opposite sides of the disc. In a serial embodiment, one side of         the disc is cleared and then the other side is cleared. In a         simultaneous embodiment, the two sides of the disc are cleared         simultaneously.

The working channel can be straight or bent. Also, the cross sectional area of the working channel can vary (e.g., it can be a funnel-shaped working channel).

In use, in some embodiments, the distal end portion of the tool can be swept side-to-side without longitudinal movement. In other embodiments, the distal end portion of the tool can be swept side-to-side with simultaneous unidirectional longitudinal movement. In other embodiments, the distal end portion of the tool can be swept side-to-side with simultaneous longitudinal back-and-forth movement.

In some embodiments, the tool of the present invention is used to clear a disc. In others, it is used to clean disc endplates abutting the disc. In still others, it is used to both clear a disc and clear its associated endplates.

Now referring to FIGS. 6-8, one goal of the present invention is to guide and steer a rotating, curved flexible transmission shaft that has to transmit an incoming Torque M_(in) (from the first right cylindrical portion A), in order to reach an outgoing torque M_(out) at the second right cylindrical portion B, where A and B are not always concentric, but oriented against each other over a certain (variable) angle α and have a flexible, curved portion inbetween (see FIG. 6). In order to accomplish this goal, the steering cannula needs to be able to guide both straight transmission shaft portions A and B, but also to connect these guiding zones and actively change the angle between them (see FIG. 7) Merely providing the steering cannula with one or two joints between A and B would not be satisfactory because the joint geometry might interfere with the inner, rotating flexible shaft. Rather, because of the importance of optimizing the material transport and torque transmission, it is believed the inner rotating flexible shaft needs to have a smooth transition shape between portions A and B (i.e., no joints). Consistent with that desire is a belief that the steering cannula must also have a smooth transition shape in order to minimize interference and to maximize the guidance (as shown by the dotted line in FIG. 8). Merely providing a simple elastic tube as the flexible part (FP) of the cannula probably does not provide enough stability and at the same time enough flexibility.

Therefore, it is believed that the outer cannula should be stable against axial torsion, but bendable in one plane. It is further believed that the cannula geometry disclosed in FIGS. 9-11B, consisting of alternating bullet-shaped cutouts, provides these desired qualities. Adequate d et ail e d dimensioning of the cutout geometry can adjust and optimize the stiffness/stability, steering angle, steering radius of the cannula.

Referring now to FIG. 9-11C, there is provided a discectomy tool comprising:

-   -   a) a cannula 31 having a proximal end portion 33, a distal end         portion 35, and an outer surface 37; the cannula having a first         longitudinal face 38 and a second opposed longitudinal face,         wherein the first longitudinal face has a plurality of         alternating, opposed transverse cutouts 39 therealong to form a         substantially square wave 36 of the first longitudinal face,     -   b) a transmission shaft disposed in the cannula, the shaft         having a proximal end portion and a distal end portion;     -   c) a cutting tip attached to the distal end portion of the         transmission shaft.

Now referring to FIG. 11D, preferably, the cutouts 101 have a tapered distal end portion 41 so as to form a bullet shape. Also preferably, the width of the middle portion 40 of the cutout is greater than the width of either the proximal end portion 42 or the distal end portion 41.

The tool of the present invention may further include bilateral pulling strips running alongside the cannula to steer the tool. In the intradiscal environment, the active steering force needs to be high (depending on the cutting ability and the resistance within the tissue). Now referring to FIGS. 12-17, in one preferred embodiment, pulling strips 43 are arranged along both the left and the right side of the cannula, and are guided through notches 45. When pulled on the left string, the flexible steering cannula (SC) will bend to the left side, and vice versa (see FIG. 12). In some embodiments, the pulling strip 43 can b e disposed on the inside of the steering cannula (see FIG. FIG. 13). In some embodiments, the pulling strips 43 are integrated into the cannula wall 47 via notches. The cannula wall thickness has to be as small as possible, in order to have as much cross sectional area as possible in the inner lumen (FIGS. 16-17). The “notch” embodiments allow the cannula inner lumen to be as large as possible to allow the rotating member to contain irrigation, torque transmission and the transport auger)

In other embodiments, the steering is accomplished by using a pushing force. This preferably occurs without uncontrolled deformation of the strip. The ability of the steering to accomplish these goals is due to the dovetail feature.

Referring now to FIGS. 12-19, there is provided a discectomy tool comprising

-   a) a cannula 31 having a proximal end portion 33, a distal end     portion 35, and an outer surface 37; -   b) a transmission shaft disposed in the cannula, the shaft having a     proximal end portion and a distal end portion; -   c) a cutting tip attached to the distal end portion of the     transmission shaft; -   d) first and second steering elements 43 bilaterally and     longitudinally disposed on the outer surface of the cannula.

In some embodiments, longitudinal notches 45 are provided on the inner steering cannula wall 49 (see FIG. 14). This embodiment, however, may be a production challenge, since EDM or profile extrusion are expensive. To have the notch on the outside surface 37 of the cannula (see FIG. 15) would offer an easier manufacturing. Moreover, its lever ratio would be more beneficial (more side bending force at the same pulling strength).

Referring now to FIG. 15, there is provided a discectomy tool comprising:

-   a) a cannula having a proximal end, a distal end, and an outer     surface having a first longitudinal recess (notch) therein; -   b) a transmission shaft disposed in the cannula, the shaft having a     proximal end portion and a distal end portion; -   c) a cutting tip attached to the distal end portion of the     transmission shaft; -   d) a first steering element disposed in the first longitudinal     recess of the outer surface.

Without wishing to be tied to a theory, it is believed that simply making the notch 45 a rectangular shape would easily allow the pulling strings to fall out. Therefore, one solution regarding steering force and manufacturing possibilities is a dovetail-like profile notch that is easily retains the like-shaped steering strip. More generally, the dovetail is one example in which the first longitudinal recess has a transverse opening at the outer surface and the steering element has a maximum transverse cross-section, wherein the maximum cross-section is greater than the opening of the recess at the outer surface, thereby preventing expulsion of the steering element from the recess.

Now referring to FIGS. 20-22, in some embodiments, the steering elements are flat metallic/polymeric tapered strips 51. They have the mechanical property of being very flexible in up-down bending, but quite stable in lateral bending. This lateral bending stability of the steering strips is essential to increase the torsional stability of the outer cannula assembly. A further advantage of flat steering strips is that the wall thickness of the outer cannula can be very small. Such steering strips have the big advantage that simultaneous pulling on one string and pushing on the other string increases the steering force and keeps the length of the neutral axis of the steering cannula constant in length if it is steered. In contrast, in many other conventional tools, steering is limited to only pulling and uses only a single pulling strip. Thus, in the present invention, there is much more defined motion, which is important in navigation.

A common challenge in discectomies is to not only to cut and detach disc material from the disc proper, but also to transport the excised disc material automatically out of the body. It is important to prevent the tissue from clogging the tool. Auger systems (modeled after the Archimedes pump shown in FIG. 23) have been proposed to achieve these goals. However, wishing to be tied to a theory, it is believed that a simple Archimedes pump itself might not be sufficient to guarantee a proper transport of the cut disc material fragment, as the disc material might get dry and stick to the auger walls, which finally leads to a clogging of several fragments and a transport interruption.

In order to prevent this clogging, and now referring to FIG. 24, some embodiments of the present invention integrates irrigation flow from an irrigation flow means (not shown) into the tool. The inflow can be performed either between the outer cannula of the disc removal device and the working channel, or through a central lumen inside the auger. With irrigation through an inner lumen, the exit point of the inflow can be located close to the cutting blade or from within the cutting blade. This allows having a permanent inflow over the blade geometries during the cutting, and helps to keep the blade clean. If the blade can not be kept clean, especially sharp edges can get covered with disc tissue and decrease the cutting ability of the blade.

It is believed that a smooth continuous transport geometry without sudden transitions is desirable to reach a reliable transport of the cut disc material fragments. However, in use, in the steerable area of the transmission shaft, the bending radius can be below 15 mm. Thus, it is desired to provide a tool that provides small bending radii, smooth auger geometry transition and adequate torque transmission. One solution is to provide a flexible torque transmission shaft overlayed by, but not directly connected to, a flexible auger element. It is believed that if directly connected at the flexible/steerable area, the construct would lose a certain amount of its flexibility, so transitional movements between the flexible torque transmission shaft and the flexible auger do have to be possible. The “loose” auger avoids this problem.

However, the flexible auger is preferably connected with the cutting blade on its distal end, and with the straight/stiff threaded shaft on its proximal end, and allows very smooth geometrical transitions between these different elements in order to prevent obstacles for a reduced resistance tissue/material transport from the cutting blade along to the auger flanks.

The flexible auger portion can comprise either:

-   -   a) polymeric or metal spiral alone, in case of very low torque         transmission: (see FIG. 25);     -   b) a polymeric spiral connected to a tube which is connected to         the inner shaft at an extension, but not along the         flexible/steering zone (decreases bending stiffness), in case of         higher torque transmission (see FIG. 26))

Now referring to FIG. 25, there is provided a discectomy tool comprising:

-   a) a cannula having an outer surface having a longitudinal bore     therein, a proximal end and a distal end; -   b) a transmission shaft having a proximal end portion and a distal     end portion; -   c) a rotatable cutting tip having a proximal end portion; and -   d) a flexible helical auger 101 having a proximal end portion 103     and a distal end portion 105, wherein the proximal end portion of     the auger is connected to the transmission shaft, and wherein the     distal end portion of the auger is connected to the proximal end     portion of the cutting tip     wherein the transmission shaft and auger are adapted to rotate     within the longitudinal bore of the cannula,     wherein the cutting tip extends out the bore at the distal end of     the cannula.

In some embodiments, the auger can be manufactured by attaching a flexible (e.g., PEEK) auger to a metal (preferably threaded) transmission shaft.

Referring now to FIG. 26, there is provided a discectomy tool comprising:

-   a) a cannula having an outer surface having a longitudinal bore     therein, a proximal end and a distal end; -   b) a transmission shaft having a proximal end portion and a distal     end portion; -   c) a rotatable cutting tip attached to the distal end portion of the     transmission shaft; and -   d) a flexible helical auger having a proximal end portion, and     intermediate portion and a distal end portion, wherein the     intermediate portion is loosely wrapped around the flexible torque     transmission shaft, wherein the proximal end portion of the auger is     connected to the transmission shaft, and wherein the distal end     portion of the auger is connected to the proximal end portion of the     cutting tip,     wherein the transmission shaft and auger are adapted to rotate     within the longitudinal bore of the cannula, wherein the cutting tip     extends out the bore at the distal end of the cannula.

The loose auger in this flexible torque transmission shaft can be provided in a number of ways, such as the following non-limiting examples:

-   -   a) narrow spring,     -   b) solid tube shaft of flexible material,     -   c) solid metal shaft with specific cutouts to become flexible in         bending etc.)

For the outflow, a suction device can be connected with the auger/transport lumen so that a continuous liquid flow is helping to transport the cut disc material fragments.

In some embodiment, the tool has a safety housing to prevent cutting of anatomic elements outside of the intervertebral disc, as, for example, the endplates of the adjacent vertebrae. With a safety housing, the blade is only able to cut to one side, and not to progress in depth. After an initial cylindrical hole is drilled (with a standard drill), the tip of the disc removal device can be inserted until it touches the ground of the hole. After this, the tip can only be steered to one direction. This means that if the depth of the initial drilled hole determines the reachable area of the cutting tip. 

We claim:
 1. A discectomy tool comprising: a) a cannula having a longitudinal bore, an outer surface having a first longitudinal recess therein, a proximal end portion and a distal end portion, b) a first steering element longitudinally disposed in the first longitudinal recess of the outer surface and extending in the direction of the longitudinal bore, c) a flexible, hollow transmission shaft disposed in the cannula, the shaft having a throughbore, a proximal end portion, a distal end portion and an outer surface having a thread extending therefrom, d) an irrigation source fluidly connected to the throughbore, and e) a cutting tip attached to the distal end portion of the transmission shaft, wherein the first longitudinal recess defines an opening along the outer surface and the first steering element has a maximum cross-section, wherein the maximum cross-section is greater than a width of the opening of the first longitudinal recess at the outer surface to prevent radial removal of the first steering element from the first longitudinal recess, wherein the cannula has longitudinally repeating cutouts to produce increased flexibility, and wherein axial movement of the first steering element relative to the cannula induces flexion of the cannula.
 2. The tool of claim 1 wherein the repeating cutouts form a square wave in a face of the cannula.
 3. The tool of claim 1 wherein the shaft and cutting tip extend through the distal end of the cannula.
 4. The tool of claim 1, wherein the cannula has a second longitudinal recess in its outer surface, the tool further comprising a second steering element disposed in the second longitudinal recess of the outer surface.
 5. The tool of claim 1, wherein the first longitudinal recess and the first steering element have matching dovetail cross-sections.
 6. The tool of claim 1, wherein the first steering element is a flat metallic or polymeric strip.
 7. The tool of claim 1, wherein the first steering element is a flat metallic or polymeric tapered strip.
 8. The discectomy tool of claim 1, comprising: the first steering element and the second steering element bilaterally and longitudinally disposed with respect to the cannula.
 9. The discectomy tool of claim 1, comprising: the cannula includes a first longitudinal face and a second opposed longitudinal face, and wherein the first longitudinal face includes the cutouts, and wherein the cutouts define a plurality of alternating, opposed transverse cutouts along the first longitudinal face to form a substantially square wave of the first longitudinal face.
 10. The tool of claim 9 wherein the first steering element comprises a first steering wire disposed in the first longitudinal recess of the outer surface of the cannula substantially along a single bending plane.
 11. The tool of claim 10 further comprising a second steering wire contacting the outer surface of the cannula substantially along the single bending plane.
 12. The tool of claim 11 wherein the cutouts have a tapered distal end portion so as to form a bullet shape.
 13. The tool of claim 11 wherein at least one of the cutouts has a proximal end portion, a middle portion and a distal end portion, each portion having a width, and wherein the width of the middle portion of the cutout is greater than the width of either the proximal end portion or the distal end portion. 